Studies of interferon (IFN)-induced receptor mediated gene expression led to the initial discovery of a Janus kinase (JAK) signaling pathway, which has been shown to be a common signaling pathway used by many cytokines and growth factors. The mammalian JAK family of intracellular tyrosine kinases, has four members; JAK1, JAK2, JAK3 and Tyk2. JAKs range in size from 120 to 140 kDa and contain seven conserved JAK homology (JH) domains which define this kinase super family.
Prototypically, the binding of a cytokine to its cell surface receptor results in receptor dimerization and subsequent activationphosphorylation of JAK tyrosine kinases which are constitutively associated with the receptor. Specific tyrosine residues on the receptor are then phosphorylated by activated JAKs and serve as docking sites for a family of latent cytoplasmic transcription factors known as Signal Transducers and Activators of Transcription (STATS). STATS are phosphorylated by JAKs, dimerize, then translocate to the nucleus where they bind specific DNA elements and activate gene transcription.
Many pro-inflammatory cytokines (IL-6, IL-12, IL-15, IL-23, GM-CSF and IFN-γ) which are implicated in autoimmune diseases mediate their activity through the JAK kinases. As a consequence, these enzymes have long been considered attractive drug targets. The essential role of JAKs in mediating the biological effects of cytokines has been confirmed by natural mutations in humans and targeted disruption in mice. Humans with a genetic loss of JAK3 have a severe combined immunodefiency (SCID) phenotype due to a developmental block in T and NK cell development and nonfunctional B-cells. Humans lacking Tyk2 are susceptible to microbial infection, have a Th2 bias with Hyper-IgE syndrome and defective cytokine signaling (IL-6, 10, 12 and 23). Signaling can be restored by transfection of the wild type kinase.
Animal KO models of the JAK family of kinases have demonstrated significant phenotypes. JAK1 KO animals exhibit defective responses to class 2 cytokines (IL-10 family), those utilizing the common gamma chain γc (IL-2, IL-4 etc) and gp130 receptor subunits (IL-6, LIF, OSM), resulting in perinatal lethality due to developmental, neurological and lymphoid defects. JAK2 KO mice exhibit defective erythropoiesis caused by a block in EPO signaling, resulting in embryonic lethality. JAK3 KO mice are viable but exhibit a SCID phenotype with nonfunctional T-cells and a lack of B and NK-cells (similar to human mutation). Tyk2 KO animals manifest modest viral susceptibility, reduced IL-12 responses, resistance to arthritis and enhanced Th2 cell-mediated allergic inflammation.
A considerable body of literature has accumulated that link the JAK/STAT pathway to various diseases and disorders including hyperproliferative disorders and cancer such as leukemia and lymphomas, immunological and inflammatory disorders such as transplant rejection, asthma, chronic obstructive pulmonary disease, allergies, rheumatoid arthritis, type I diabetes, amyotropic lateral sclerosis and multiple sclerosis.